Horizontal parallelogram correction combined with horizontal centering

ABSTRACT

An electron beam tends to slope downwardly as it is deflected horizontally to form a raster in a video display apparatus. The sloping of the beam can cause geometric errors in the raster, for example orthogonality and parallelogram errors. A raster correction circuit substantially offsets the downward slope of the electron beam by modulating a vertical deflection current with an induced horizontal-rate raster correction current, thereby substantially eliminating orthogonality and parallelogram errors in the raster. A raster correction transformer utilizes a raster centering inductor for a primary winding, and a horizontal-rate centering current is magnetically coupled into the vertical deflection coils to modulate the vertical deflection current.

BACKGROUND

This invention relates generally to the field of raster correctioncircuits, and, in particular, to correction of orthogonality andparallelogram errors in a raster of a cathode-ray tube of a videodisplay apparatus.

A deflection system utilized in a video display apparatus, such as atelevision receiver or a video display monitor, typically includescircuitry that allows for the adjustment of a raster on the viewingscreen of the apparatus's cathode-ray tube. Such circuitry is requiredbecause of, among other things, the nature of the scanning process andthe geometry of the cathode-ray tube.

For example, such circuitry may include a raster correction circuit foreliminating orthogonality and parallelogram errors in the scannedraster. The nature of the orthogonality and parallelogram errors and anapproach to eliminating both of them is described in a U.S. patentapplication entitled “VERTICAL DEFLECTION CIRCUIT WITH RASTERCORRECTION”, which was filed on May 17, 1996, in the name of WalterTruskalo et al., and was assigned Ser. No. 08/649,409. That applicationdiscloses an arrangement for modulating a vertical deflection current ata horizontal rate for substantially offsetting a downhill scan effectcaused by vertical deflection of the electron beam, thereby correctingorthogonality and parallelogram errors in the raster. A raster subjectto orthogonality and parallelogram errors is illustrated in FIG. 1.

Such circuitry may also include a centering circuit for, illustratively,horizontally centering the raster on the viewing screen of the tube.Centering the raster is necessary to ensure the most efficient use ofthe tube, which occurs when the size of the scanned raster issubstantially the same size as the tube's viewing screen. The need forhorizontal centering is most pronounced when the amount of horizontaloverscan is reduced or, in other words, when the size of the scannedraster is reduced to the size of the tube's viewing screen. Centeringthe raster is typically accomplished by causing a direct current ofselected polarity and amplitude to flow through the appropriatedeflection coils, either horizontal or vertical.

In the manufacture of a video display apparatus, it is desirable toconsolidate circuitry to the greatest possible extent. Suchconsolidation provides several advantages, among them: decreased partscount, decreased cost, increased reliability, and an increase in theamount of space available within the apparatus's chassis. Accordingly,it is desirable to consolidate the circuits that perform the rastercorrection and centering functions.

SUMMARY

The present invention is directed to a deflection system that satisfiesthe need to consolidate circuitry in a video display apparatus to thegreatest possible extent.

A deflection system according to the inventive arrangements taughtherein comprises a vertical deflection coil for deflecting the scanningelectron beam between upper and lower edges of the raster; a rastercentering circuit, which has a centering inductor, for centering theraster on the screen; and a raster correction transformer. The rastercorrection transformer uses the centering inductor for a primary windingand has a secondary winding coupled to the vertical deflection coil. Thecentering inductor and the secondary winding are advantageously woundaround the same core.

The vertical deflection coil may comprise first and second verticaldeflection windings coupled in either a series or a shunt arrangement.

It is advantageous to use the centering inductor as the primary windingof the raster correction transformer because then the verticaldeflection circuit and the raster centering circuit can both be mountedwith the deflection yoke assembly on a neck portion of the cathode-raytube of the video display apparatus. This simplifies assembly of thevideo display apparatus because it obviates the need to run wires fromthe chassis of the video display apparatus to the vertical deflectioncircuit and the raster centering circuit.

The above, and other features, aspects, and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencenumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is useful for explaining orthogonality and parallelogram errorsin a raster;

FIGS. 2 and 3 show diagrams, in block and schematic form, of deflectionsystems for video display apparatus according to inventive arrangementsdescribed herein;

FIGS. 4a-4 f shows voltage and current waveforms useful for explainingthe operation of the deflection systems of FIGS. 2 and 3;

FIG. 5 is a diagram, in block and schematic form, of a first equivalenthorizontal centering circuit for the deflection systems of FIGS. 2 and3;

FIG. 6 shows a voltage waveform characteristic of the equivalentcentering circuit of FIG. 5;

FIG. 7 is a diagram, in block and schematic form, of a second equivalenthorizontal centering circuit for the deflection systems of FIGS. 2 and3;

FIG. 8 shows a voltage waveform characteristic of the equivalenthorizontal centering circuit of FIG. 7; and

FIGS. 9 and 10 show current waveforms characteristic of the deflectionsystem of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An inventive embodiment of a deflection system 400 for a video displayapparatus, such as a television receiver or a video display monitor, isshown in FIG. 2. A horizontal deflection circuit 100 and a verticaldeflection circuit 200 cooperatively deflect a scanning electron beam toform a raster on a screen of the video display apparatus. The horizontaldeflection circuit 100 deflects the scanning electron beam across thescreen at a horizontal scanning rate. Simultaneously, the verticaldeflection circuit 200 deflects the electron beam downwardly at aslower, vertical scanning rate. A raster centering circuit 300 derivesenergy from the horizontal deflection circuit 100 in order tohorizontally center the scanned raster on the screen of the videodisplay apparatus. In order to consolidate circuitry within the videodisplay apparatus, the vertical deflection circuit 200 advantageouslyuses a horizontal centering inductor L_(C) of the raster centeringcircuit 300 as a primary winding of a raster correction transformer 41.

The embodiments described herein employ horizontal deflection, orscanning, rates equal to approximately 31,468 Hz, commonly referred toas the “2H” scanning frequency. It will become apparent to those skilledin the art that the inventive arrangements described herein are notlimited to any particular horizontal or vertical deflection frequencies,but can be utilized throughout the entire range of useful horizontal andvertical deflection frequencies.

Voltage and current waveforms associated with the horizontal deflectioncircuit 100 are shown in FIGS. 4a-4 f; current flow is defined aspositive in the directions indicated in FIG. 2. Referring to FIG. 2, aB⁺ voltage of approximately 140 V_(dc) is impressed across anS-correction capacitor C_(S) through a primary winding L_(PRI) of ahigh-voltage transformer IHVT. As an electron beam is deflected to anupper left-hand corner of the raster, a horizontal output transistor Q1does not conduct a current. Energy previously stored in a horizontaldeflection coil L_(H) causes a current to flow through a forward-biaseddamper diode D1 and the horizontal deflection coil L_(H) and into theS-correction capacitor C_(S). At this point both a damper current I_(D)and a horizontal deflection current I_(H) attain their peak negativevalues.

When the scanning electron beam reaches the center of the raster, theenergy stored in the horizontal deflection coil L_(H) has decayed tozero and the horizontal deflection current I_(H) and the damper currentI_(D) are equal to approximately zero. The damper diode D1 becomesreverse biased and a horizontal deflection oscillator 10 causes thehorizontal output transistor Q1 to conduct a current I_(HOT). Thehorizontal deflection current I_(H) reverses direction, and energysupplied to the horizontal deflection coil L_(H) by the S-correctioncapacitor C_(S) allows the horizontal deflection current I_(H) toincrease linearly.

When the scanning electron beam reaches the right edge of the raster,the horizontal deflection oscillator 10 causes the horizontal outputtransistor Q1 to discontinue conducting the current I_(HOT) and thedamper diode D1 remains reverse biased. During this retrace interval,the decaying horizontal deflection current I_(H) flows rapidly into theretrace capacitor C_(R). When horizontal deflection current I_(H) decaysto approximately zero, it reverses direction and is then supplied byretrace capacitor C_(R). Once the retrace capacitor C_(R) has dischargedits stored energy through the horizontal deflection coil L_(H), theelectron beam has been returned to the upper left-hand corner of theraster, and the process repeats.

In the vertical deflection circuit 200 shown in FIG. 2, a vertical-ratesawtooth generator 61 provides a vertical-rate sawtooth waveform to anon-inverting input of a vertical output amplifier 62. The verticaloutput amplifier 62 is coupled to a positive supply voltage, for example+24 V, and a negative supply voltage, for example a ground potential,and may comprise a complementary or quasi-complementary push-pulltransistor output stage. The vertical output amplifier 62 drives firstand second vertical deflection windings L_(V1) and L_(V2) of a verticaldeflection coil with a vertical-rate sawtooth current I_(V). Thevertical deflection windings L_(V1) and L_(V2) are coupled in a seriesarrangement; the current flowing through these windings may have apeak-to-peak amplitude equal to approximately 2 A. A voltage dividerformed by resistors R3 and R4 generates a feedback voltage, which iscoupled to the inverting input of the vertical output amplifier 62through a resistor R5. A capacitor C3 provides S correction for thevertical deflection current I_(V).

A series arrangement of resistors R1 and R2 and a potentiometer P1 iscoupled in parallel with the two vertical deflection windings L_(V1) andL_(V2). The resistors R1 and R2 and the potentiometer P1 are selectedduring the design of a deflection yoke for the cathode-ray tube, andthese resistances are included as part of a deflection yoke assembly.The three resistances are used to adjust the convergence of the electronbeams within the cathode-ray tube. The potentiometer P1 is adjusted toachieve a desired crossover of the electron beams from the outerelectron guns, typically red and blue, at a vertical center line of thecathode-ray tube.

In a presently preferred embodiment of an inventive arrangementdescribed herein the horizontal deflection circuit 100 combines with avertical deflection circuit 200′ to form a deflection system 400′, whichis shown in FIG. 3. In the vertical deflection circuit 200′, thevertical deflection windings L_(V1) and L_(V2) are coupled in a shuntarrangement; the shunt arrangement is advantageously used in order toobtain a shorter vertical retrace time and to enable a lower inductancefor the vertical deflection coil for the same applied voltage. Thecoupling of the secondary winding of transformer 41 to the first andsecond vertical deflection windings L_(V1) and L_(V2) does not disturbthe shunt nature of the arrangement of the vertical deflection windingsL_(V1) and L_(V2). The peak-to-peak amplitude of currents I′_(LV1) andI′_(LV2) flowing through each of the vertical deflection windings mayhave a peak-to-peak amplitude equal to approximately 2 A. A feedbackvoltage is generated across a resistor R8 and is coupled to theinverting input of the vertical output amplifier 62 by a resistor R9.Resistors R6 and R7 and a capacitor C4 provide a damping network for thedeflection windings L_(V1) and L_(V2).

The raster centering circuit 300 of FIGS. 2 and 3 comprises a horizontalcentering inductor L_(C), a centering capacitor C_(C), diodes D2 and D3,a switch device S1, and a variable resistance P2, which may comprise apotentiometer. The horizontal centering inductor L_(C) has, for example,N1 turns and typically has a greater inductance, and hence conducts alower peak-to-peak current, than does the horizontal deflection coilL_(H). The switch device S1 may comprise, for example, a slide switch ora single-pole, double-throw rotary switch of the type disclosed in U.S.Pat. No. 4,703,233, issued on Oct. 27, 1987, to E. Rodriguez-Cavazos.

As shown in FIGS. 2 and 3, the centering circuit 300 derives energy fromthe horizontal deflection circuit 100. For purposes of the presentdescription, the switch device S1 makes a connection with the anode ofthe diode D3 to provide an equivalent centering circuit 300′, which isshown in FIG. 5. Referring to FIG. 5, during a negative portion of thehorizontal deflection current I_(H), which corresponds to the flow ofthe damper current I_(D) through the horizontal deflection coil L_(H),and, thus, to deflection of the electron beam from the left edge to thecenter of the raster, the diode D2 is reverse biased, the diode D3 isforward biased, and the horizontal-rate centering current I_(C) chargesthe S-correction capacitor C_(S) through the diode D3. A small, positivecentering voltage V′_(C), clamped to approximately the sum of theforward voltage drop of the diode D3, is established across thecentering capacitor C_(C), as shown in FIG. 6, and a negative portion ofthe horizontal-rate centering current I_(C) flows through the horizontalcentering inductor L_(C).

As the electron beam reaches the center of the raster, the horizontaldeflection current I_(H) reverses direction and becomes positive, whichcorresponds to the flow of the current I_(HOT) through the horizontaldeflection coil L_(H) and, thus, to deflection of the electron beam fromthe center to the right edge of the raster. The horizontal-ratecentering current I_(C) also becomes positive. The diode D2 is nowforward biased, the diode D3 is now reverse biased, and ahorizontal-rate current flows through the diode D2 and the variableresistance P2. The centering voltage V′_(C) becomes negative, as shownin FIG. 6, and is equal to approximately the voltage V_(P2) generatedacross the variable resistance P2.

The successive magnitudes of the negative peaks of the centering voltageV′_(C) produce an average voltage V′_(avg), as shown in FIG. 6. Thevoltage V′_(avg) generates a positive component of the horizontal-ratecentering current I_(C) flowing through the horizontal centeringinductor L_(C).

Setting the switch device S1 to make its connection to the anode of thediode D3 may prove to be inadequate to center the raster properly on theface of the cathode-ray tube. In that event, the switch device S1 isadjusted to make its connection to the cathode of the diode D1 toprovide an equivalent centering circuit 300″, which is shown in FIG. 7.The circuit of FIG. 7 operates similarly to the circuit of FIG. 5, withthe exception that the voltages provided across the centering capacitorC_(C) in the two circuits have opposite polarities.

Referring to FIG. 7, during a negative portion of the horizontaldeflection current I_(H), which corresponds to the flow of the dampercurrent I_(D) through the horizontal deflection coil L_(H) and, thus, todeflection of the electron beam from the left edge to the center of theraster, a negative portion of the horizontal-rate centering currentI_(C) flows through the horizontal centering inductor L_(C). The diodeD2 is reverse biased, the diode D3 is forward biased, and thehorizontal-rate centering current I_(C) charges the S-correctioncapacitor C_(S) through the variable resistance P2 and the diode D3. Apositive centering voltage V″_(C) is established across centeringcapacitor C_(C), as shown in FIG. 8, and is equal to approximately thevoltage V_(P2) generated across the variable resistance P2.

The successive magnitudes of the positive peaks of the centering voltageV″_(C) produce an average voltage V″_(avg), which is shown in FIG. 8.The voltage V″_(avg) generates the horizontal-rate centering currentI_(C) through the horizontal deflection coil L_(H) in the same directionas the damper current I_(D).

As the electron beam reaches the center of the raster, the horizontaldeflection current I_(H) reverses direction and becomes positive, whichcorresponds to the flow of the current I_(HOT) through the horizontaldeflection coil L_(H) and, thus, to deflection of the electron beam fromthe center to the right edge of the raster. The horizontal-ratecentering current I_(C) also becomes positive. The diode D2 is nowforward biased, the diode D3 is now reverse biased, and ahorizontal-rate current flows through the diode D2. The centeringvoltage V″_(C) becomes negative, as shown in FIG. 8, and is clamped toapproximately the sum of the forward voltage drop of diode D2.

In accordance with an inventive arrangement described herein, thedeflection systems 400 and 400′ shown in FIGS. 2 and 3, respectively,advantageously use the horizontal centering inductor L_(C) of the rastercentering circuit 300 as the primary winding of the raster correctiontransformer 41. In the presently preferred embodiment shown in FIG. 3,the horizontal centering inductor L_(C) has 380 turns. The secondarywinding of the transformer 41 is coupled in series with the first andsecond vertical deflection windings L_(V1) and L_(V2) and has 16 turns.A center-tap 47 divides the secondary winding into a first winding 43 aand a second winding 43 b, each of which has 8 turns. The particularnumber of primary and secondary turns of the raster correctiontransformer 41, and hence its turns ratio, is dependent upon therequirements of a particular deflection system and is left to thejudgment of one skilled in the art.

Both the horizontal centering inductor L_(C) and the first and secondwindings 43 a and 43 b are advantageously wound around the same core,for example a ferrite rod core which, in a presently preferredembodiment, has a diameter of approximately 0.399 inches and a length ofapproximately 1 inch. The use of a rod core is illustrative, and is notintended to suggest that a core configuration which has a closed-loopmagnetic path length, for example a toroid, cannot be used. Asignificant factor for one skilled in the art to take into account whenselecting a particular core is the need to avoid saturating the corewith the horizontal rate centering current I_(C) flowing through thehorizontal centering inductor L_(C) and with vertical currents I_(LV1)and I_(LV2) (in a series arrangement) and I′_(LV1) and I′_(LV2) (in ashunt arrangement) flowing through the first and second verticaldeflection windings L_(V1)and L_(V2); such saturation can causeundesirable distortions in the parallelogram correction currents.

It is advantageous to use the horizontal centering inductor L_(C) as theprimary winding of the raster correction transformer 41 because then thevertical deflection circuit 200 or 200′ and the raster centering circuit300 can both be mounted with the deflection yoke assembly on a neckportion of the cathode-ray tube of the video display apparatus. Thissimplifies assembly of the video display apparatus because it obviatesthe need to run wires from the chassis of the video display apparatus tothe vertical deflection circuit 200 or 200′ and the raster centeringcircuit 300.

In the embodiments shown in FIGS. 2 and 3, the horizontal deflectioncircuit 100 generates a horizontal deflection voltage V_(Q1), which isshown as FIG. 4b and typically has a peak-to-peak voltage which isapproximately equal to 1200 V. The horizontal deflection voltage V_(Q1)is stepped down in accordance with the turns ratio of raster correctiontransformer 41, which is equal to {fraction (N2/N1)}. The resultingstepped-down, horizontal-rate pulse waveform is divided substantiallyequally between the first and second windings 43 a and 43 b. Forexample, in the presently preferred embodiment of FIG. 3, thestepped-down horizontal-rate pulse waveform has a peak-to-peak voltageof approximately 28 V and is divided substantially equally across firstand second windings 43 a and 43 b of secondary winding 43. Thus, firstand second windings 43 a and 43 b are each provided with ahorizontal-rate pulse waveform which has a peak-to-peak voltage ofapproximately 14 V.

Returning to the deflection system 400 of FIG. 2, the stepped-downhorizontal-rate pulse waveforms across the first and second windings 43a and 43 b induce the horizontal-rate raster correction currents I_(LV1)and I_(LV2), respectively, for the first and second vertical deflectionwindings L_(V1) and L_(V2). The raster correction currents I_(LV1) andI_(LV2) are not constrained to have equal peak-to-peak amplitudes byvirtue of the center-tap 47. In addition, the peak-to-peak amplitudes ofthe raster correction currents I_(LV1) and I_(LV2) may vary as thecoupling between the horizontal centering inductor L_(C) and thesecondary winding of transformer 41 changes for different choices of theferrite core.

In the deflection system 400′ of FIG. 3, the stepped-downhorizontal-rate pulse waveforms across the first and second windings 43a and 43 b induce the horizontal-rate raster correction currentsI′_(LV1) and I′_(LV2), as shown in FIGS. 9 and 10. The raster correctioncurrents I′_(LV1) and I′_(LV2) are not constrained to have equalpeak-to-peak amplitudes by virtue of the shunt arrangement of windingsL_(V1) and L_(V2). In addition, the peak-to-peak amplitudes of theraster correction currents I′_(LV1) and I′_(LV2) may vary as thecoupling between the horizontal centering inductor L_(C) and thesecondary winding 43 changes for different choices of the ferrite core.

The raster correction currents I_(LV1) and I_(LV2) (in a seriesarrangement) and I′_(LV1) and I′_(LV2) (in a shunt arrangement) flowthrough the first and second vertical deflection windings L_(V1) andL_(V2), respectively, in a direction such that a magnetic field iscreated which opposes the downhill scan effect. In this way the verticaldeflection current is modulated at a horizontal rate and the downhillscan effect is substantially offset for each horizontal scanning line ofthe raster.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

What is claimed is:
 1. A deflection system for forming a raster on ascreen of a video display apparatus, said deflection system comprising:a vertical deflection coil for deflecting said scanning electron beambetween first and second edges of said raster; a raster centeringcircuit for centering said raster on said screen, said centering circuitcomprising a centering inductance; and a raster correction transformerhaving said centering inductance for a primary winding and a secondarywinding coupled to said vertical deflection coil.
 2. The deflectionsystem of claim 1, wherein said vertical deflection coil comprises firstand second vertical deflection windings.
 3. The deflection system ofclaim 2, wherein said first and second vertical deflection windings arecoupled in a series arrangement.
 4. The deflection system of claim 2,wherein said first and second vertical deflection windings are coupledin a shunt arrangement.
 5. The deflection system of claim 2, whereinsaid secondary winding is coupled in series with said first and secondvertical deflection windings.
 6. The deflection system of claim 5,further comprising a series interconnection of a plurality ofresistances wherein one of said plurality has a terminal coupled to oneof said vertical deflection windings and another of said plurality has aterminal coupled to the other of said vertical deflection windings. 7.The deflection system of claim 6, wherein one of said plurality ofresistances comprises a potentiometer.
 8. The deflection system of claim7, wherein: said secondary winding comprises a center-tap; and saidcenter-tap is coupled to a wiper of said potentiometer.